The present invention relates to a coated polyester film superior in ink adhesive property, printability and resistance to falling off of particles from a coating layer, which is particularly preferable as a substrate for labels.
Polyester films have been applied to a wide variety of uses because of their high crystallinity and superiority in transparency, gloss, mechanical property and chemical resistance. For imparting functionality, such as printability, blocking resistance, antistatic property and the like, a coating layer is generally formed on the surface of a polyester film. For achieving both the transparency and handling property (blocking resistance, slip property, wear and abrasion resistance and the like), it is a general practice to add particles to a coating layer of a polyester film, thereby to form irregularities on the film surface.
In the seal printing frequently used particularly for label printing and offset printing, however, the roll of a printer slips on the print surface and the like and rubs the film. As a result, the film is scraped and particles of the coating layer fall in a powder from the surface of the coating layer. While the falling off of particles from the coating layer has not been noted heretofore, it leads to the loss of the above-mentioned properties and further to staining of the roll. Moreover, deinking occurs to degrade the product value.
It is therefore an object of the present invention to solve the above-mentioned problems and provide a coated polyester film superior in resistance to falling off of particles from a coating layer and printability, while maintaining the ink adhesive property.
According to the present invention, it has been found that specifying of the specular glossiness of the surface of the coating layer of a polyester film leads to the production of a coated polyester film having satisfactory ink adhesive property, printability and resistance to falling off of particles from a coating layer.
The object of the present invention can be achieved by the following.
1) A coated polyester film comprising a polyester film having opacity as shown by an optical density of not less than 0.3, and a coating layer formed on at least one surface of said polyester film, wherein a surface of the coating layer shows a 60-degree specular glossiness G1 and a 75-degree specular glossiness G2 that satisfy the following formulas (1) and (2):
G1xe2x89xa620xe2x80x83xe2x80x83(1) 
1 less than G2/G1xe2x89xa64xe2x80x83xe2x80x83(2) 
2) The coated polyester film of the aforementioned 1), wherein the polyester film has voids in the film and an apparent density of 0.3xe2x88x921.3 g/cm3.
3) The coated polyester film of the aforementioned 1), wherein the aforementioned coating layer is made from a composition comprising, as main components, at least one resin selected from polyester, polyurethane, acrylic polymer and copolymers thereof, and one or more kinds of inert particles.
4) The coated polyester film of the aforementioned 1), wherein the aforementioned resin comprises a water-insoluble and water-dispersible polyester resin and a water-soluble polyurethane resin having at least one block isocyanate in a molecule.
5) The coated polyester film of the aforementioned 1), wherein a surface of the aforementioned coating layer has a surface resistivity of not more than 1xc3x971013 xcexa9/xe2x96xa1.
6) The coated polyester film of the aforementioned 1), wherein the polyester film has voids in the film and wherein a ratio of the number of voids to a film thickness is not less than 0.20 void/xcexcm.
The polyester resin, which is a main component of the polyester film to be used for a substrate in the present invention, comprises a polyester obtained by polycondensation of an aromatic dicarboxylic acid, such as terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid and the like or an ester thereof, and glycol, such as ethylene glycol, diethylene glycol, 1,4-butanediol, 1,6-hexamethylene glycol, neopentyl glycol and the like.
The polyester resin may contain copolymerizable aromatic, aliphatic or alicyclic dicarboxylic acid and aromatic, aliphatic or alicyclic glycol as components.
Such polyester resin can be produced by polycondensation of aromatic dicarboxylic acid and glycol after esterification, polycondensation of aromatic dicarboxylic acid alkyl ester and glycol after transesterification, polycondensation of aromatic dicarboxylic acid diglycol ester, or by other known method.
Examples of the polyester resin include thermoplastic polyester resin such as polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalate, and the like. The polyester may be a homopolymer, or contain a heterologous polyester resin, or a copolymer comprising a third component. In any case, a polyester comprising ethylene terephthalate, butylene terephthalate, ethylene-2,6-naphthalate unit in a proportion of not less than 70 mol %, preferably not less than 80 mol %, more preferably not less than 90 mol %, is preferable. Of these, polyethylene terephthalate is most preferable.
The polyester film to be used in the present invention is particularly preferably a biaxially oriented film from the practical aspect of strength, stiffness and the like.
The polyester film to be used in the present invention may have a monolayer structure or a multilayer structure. The film preferably contains a layer that is partly or entirely non-transparent. The polyester film has an optical density showing the opacity of not less than 0.3, preferably 0.3-4.0, particularly preferably 0.5-3.0. When the optical density is less than 0.3, any printing on the surface of the coated polyester film obtained from such film becomes unpreferably illegible and unclear. When the optical density is not more than 4.0, more superior legibility can be expected.
The optical density within the above-mentioned range can be achieved without particular limitation by any method. For example, it is achieved by adding, to a polyester resin, inorganic particles or a thermoplastic resin incompatible with the polyester resin, without particular limitation on the content thereof. When inorganic particles are added, the content thereof is preferably 5-35 wt %, particularly preferably 8-25 wt %, of the polyester produced. When an incompatible thermoplastic resin is added, its content is preferably 5-35 wt %, particularly preferably 8-28 wt %, of the polyester. When inorganic particles and a thermoplastic resin incompatible with the polyester resin are used in combination, the total amount thereof is preferably not more than 40 wt % of the polyester film, from the aspects of film strength, stiffness and stability during film forming.
While the inorganic particles to be used are not subject to any particular limitation, those having an average particle size of 0.1-4.0 xcexcm, particularly preferably 0.3-1.5 xcexcm, are preferable. The inorganic particles are exemplified by white pigments such as titanium oxide, barium sulfate, calcium carbonate, zinc sulfide and the like, which may be used in combination upon mixing. Furthermore, inorganic particles, such as silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, calcium fluoride, calcium sulfate and the like, which are generally used for films, may be concurrently used.
While the thermoplastic resin incompatible with a polyester resin is not subject to any particular limitation, polyolefin resin such as polystyrene resin, polyethylene resin, polypropylene resin, polymethylpentene resin and the like, acrylic resin, phenoxy resin, polyphenylene oxide resin, polycarbonate resin and the like can be mixed with a polyethylene terephthalate resin. These thermoplastic resins may be used in a mixture and may be modified. It is needless to say that they can be used concurrently with the above-mentioned inorganic particles. Where necessary, various brighteners may be added.
The polyester film to be used in the present invention is preferably a microporous polyester film having an apparent density of 0.3-1.3 g/cm3.
A microporous polyester film having a ratio of the number of voids therein to the film thickness (hereinafter to be abbreviated as a void ratio) of not less than 0.20 void/xcexcm, preferably not less than 0.25 void/xcexcm, more preferably not less than 0.30 void/xcexcm, is preferable for both the cushioning property and surface peel strength. A coated polyester film obtained from such film is superior in clearness of print and processability during printing. As used herein, the void ratio (void/xcexcm) can be defined by the formula:
void number (voids) in the film thickness direction/film thickness (xcexcm) 
The upper limit of the void ratio is preferably 0.80 void/xcexcm, more preferably 0.55 void/xcexcm, in view of the void forming efficiency. The void ratio can be adjusted to fall within the above-mentioned range by changing the amount and the kind of incompatible thermoplastic resin to be added, viscosity thereof and the like. The void ratio can be also adjusted by changing the shape of a screw of an extruder, setting a static mixer in the flow path of molten resin and the like. A different method may be used for this end.
Such microporous polyester film is particularly useful because the opacity can be further improved by scattering of the light that occurs in the interface between fine voids in the film and matrix polyester, which improved opacity in turn reduces the amount of the aforementioned inorganic particles to be added. In addition, the presence of fine voids makes the substrate film itself lightweight, making handling easy and affording a greater economical effect in cutting costs of starting materials and transportation, and the like.
Such microporous polyester film can be obtained by a known method comprising kneading a thermoplastic polyester resin, which is a matrix, with a thermoplastic resin incompatible with the polyester resin, and drawing the obtained sheet, comprising the incompatible resin dispersed in a fine particle state in the polyester resin, at least in a monoaxial direction, thereby forming voids around the aforementioned incompatible resin particulates, or by other method.
The obtained microporous polyester film has a thickness of preferably 5-300 xcexcm. Particularly, a microporous polyester film having a void ratio of not less than 0.20 void/xcexcm preferably has a thickness of 20-300 xcexcm, more preferably 40-250 xcexcm.
The coated polyester film of the present invention essentially has a coating layer formed on at least one surface of a polyester film substrate. This coating layer is preferably an adhesion-improving layer having an ink residual rate by the crosscut method of not less than 90%, according to the evaluation method of the ink adhesive property to be mentioned later.
A coating layer may be formed by applying a coating solution, comprising an adhesion-improving resin composition, to the surface of a polyester film, which is a substrate, or by laminating an adhesion-improving resin on a polyester film, which is a substrate, by coextrusion, or by other method. To improve adhesion between a polyester film, which is a substrate, and a coating layer, moreover, the film may be subjected to a surface treatment in advance. The surface treatment may be, for example, a corona discharge treatment, a plasma discharge treatment, an active energy beam irradiation, such as ultraviolet (UV) irradiation treatment, electron beam (EB) irradiation treatment and the like, a flame treatment, or vapor deposition such as PVD, CVD and the like.
Particularly, a coating layer formed by applying a coating solution, containing an adhesion-improving resin composition, to the surface of a polyester film is most effective in the present invention for adhesion with a print ink layer or other coating materials.
Such coating layer is preferably made from a resin composition containing at least one member selected from polyester, polyurethane, acrylic polymer and copolymers thereof, and one or more kinds of inert particles.
A coating layer made from, of the above-mentioned adhesion-improving resins, an adhesion-improving resin containing a mixture of a water-insoluble and water-dispersible polyester resin, having a sulfone group in a molecule, and a water-soluble polyurethane resin, having at least one block isocyanate in a molecule, is particularly preferable. This is because the layer markedly improves adhesion to a substrate polyester film, as well as to printing ink, such as general ultraviolet (UV)-curable ink, oxidation polymerization type ink and the like.
In this case, the content ratio based on weight of the aforementioned water-insoluble and water-dispersible polyester resin (A) and the water-soluble polyurethane resin (B) is preferably (A)/(B)=90/10-10/90, particularly preferably (A)/(B)=80/20-20/80.
The coated polyester film of the present invention needs to have a 60-degree specular glossiness G1 and a 75-degree specular glossiness G2 of the coating layer surface, which satisfy the following formulas (1) and (2):
G1xe2x89xa620xe2x80x83xe2x80x83(1) 
1 less than G2/G1 less than 4xe2x80x83xe2x80x83(2) 
The 60-degree specular glossiness G1 of the coating layer surface needs to be not more than 20, preferably not more than 18, particularly preferably not more than 15. The ratio G2/G1 of the 75-degree specular glossiness G2 to the 60-degree specular glossiness G1 needs to be over 1 and not more than 4, preferably over 1 and not more than 3.5, particularly preferably over 1 and not more than 3.
When the 60-degree specular glossiness G1 of the coating layer surface exceeds 20, the resistance to falling off of particles from a coating layer becomes insufficient. When the ratio of the 75-degree specular glossiness G2 to the 60-degree specular glossiness G1 of the coating layer surface (G2/G1) is not more than 1, the resistance to falling off of particles becomes insufficient, and the printability is also degraded somewhat. When the G2/G1 exceeds 4, the surface of a coating layer that absorbs ink has less irregularities and shows degraded printability.
For the 60-degree specular glossiness G1 and 75-degree specular glossiness G2 of the coating layer surface to satisfy the aforementioned formulas (1) and (2), the inert particles to be contained in the coating layer are preferably adjusted to have appropriate average particle size, standard deviation of the particle size, shape (particle size ratio, longer diameter/shorter diameter), particle content, thickness of the coating layer and the like.
To be specific, the ratio of the average particle size d (xcexcm) of the inert particles, to be contained in the coating layer, to the thickness t (xcexcm) of the coating layer (d/t) is preferably 1.0-5.0, more preferably 1.2-4.0, most preferably 1.5-2.5. When d/t is less than 1.0, sufficient adhesion of ink, paper supply for printing and blocking resistance tend to be unattainable. When the ratio exceeds 5.0, the particles may fall off from the coating layer of the coated polyester film during printing and cause powder fall. The aforementioned d and t can be determined on a photograph observed with an electron microscope.
The thickness t of the coating layer is preferably 0.01-5.0 xcexcm, more preferably 0.05-1.5 xcexcm, particularly preferably 0.1-1.0 xcexcm. When the thickness of the coating layer is less than 0.01 xcexcm, the adhesion to the substrate polyester film is not sufficient, and the particles cannot be sufficiently fixed in the coating layer, resulting in falling off of particles from the coating layer. Conversely, when it exceeds 5.0 xcexcm, the particles are embedded in the resin of the coating layer, and the surface irregularities and blocking resistance, indispensable for the handling property of the coated polyester film, may not be attained.
The inert particles to be contained in the coating layer may be inorganic particles such as titanium oxide, barium sulfate, calcium carbonate, calcium sulfate, silica, alumina, talc, kaolin, clay, calcium phosphate, mica, hectorite, zirconia, tungsten oxide, lithium fluoride, calcium fluoride and the like, organic polymer particles such as polystyrene, polyacrylic, melamine, benzoguanamine, silicone resin and the like, and the like. These may be used in combination.
The inert particles in the aforementioned coating layer preferably have an average particle size d of 0.04-1.5 xcexcm, more preferably 0.06-1.0 xcexcm, particularly preferably 0.1-0.8 xcexcm. When they have an average particle size of less than 0.04 xcexcm, the surface of a coated polyester film has insufficient irregularities, which renders the ink adhesive property insufficient. When the average particle size exceeds 1.5 xcexcm, the particles may come off the coating layer of the coated polyester film and cause powder fall.
Of the aforementioned particles, inert particles having a particle size ratio (longer diameter/shorter diameter) of 1.0-1.5, and the standard deviation of particle size of not more than 1.0 are preferable from the aspect of resistance to falling off of particles from a coating layer. The inert particles having a particle size ratio (longer diameter/shorter diameter) of 1.0-1.2, and the standard deviation of particle size of not more than 0.5 are particularly preferable. The inert particles satisfying these requirements include spherical silica particles, spherical silicone resin particles, spherical crosslinked polystyrene particles, spherical crosslinked acrylic particles, spherical or cubic calcium carbonate particles, calcium phosphate particles and the like.
The average particle size, particle size ratio and standard deviation of the particle size of the inert particles in the aforementioned coating layer can be determined according to, for example, JP-A-1-284534.
The aforementioned coating layer preferably has a resin/particle weight ratio of 30/70-70/30, more preferably 35/65-60/40, particularly preferably 40/60-50/50. When the weight ratio of the resin is less than 30, the particles cannot be sufficiently fixed in the coating layer and may fall off from the coating layer to cause powder fall. Conversely, when the ratio of the resin exceeds 70, sufficient ink adhesion, paper supply property and blocking resistance are difficult to attain.
The coated polyester film of the present invention preferably has a surface resistivity of the coating layer of not more than 1xc3x971013 xcexa9/xe2x96xa1, more preferably not more than 1xc3x971012 xcexa9/xe2x96xa1. When the surface resistivity is greater than 1xc3x971013 xcexa9/xe2x96xa1, electrostatic troubles tend to occur in various steps for preparing this coated polyester film into labels, such as coating of adhesive, printing, sheet cutting, die cutting and the like. To make the surface resistivity fall within the above-mentioned range, for example, an antistatic agent may be added to the above-mentioned adhesion-improving resin composition to be mentioned below, or other method may be employed.
Particularly, the composition constituting the aforementioned coating layer preferably contains an antistatic agent, because it prevents occurrence of electrostatic troubles in various steps for preparing the obtained film into labels, such as coating of adhesive, printing, sheet cutting, die cutting and the like. Examples of the antistatic agent include those generally used as antistatic agent for coating (e.g., quaternary ammonium salt type antistatic agent), particulate carbon black, metal powder (e.g., nickel, copper and the like), metal oxide (e.g., tin oxide, zinc oxide and the like), metal coated fiber (e.g., fibrous brass, stainless, aluminum and the like), conductive filler (e.g., scaly graphite, aluminum flake, copper flake and the like), and conductive polymers (e.g., sulfonated polyaniline, polypyrrole and the like), which can be used where necessary as long as the effect of the present invention is not impaired.
As a method for forming a coating layer, a method comprising applying, as mentioned above, a coating solution, containing an adhesion-improving resin composition containing an adhesion-improving resin and inert particles as main components, to the surface of a polyester film is preferable. In this case, the liquid temperature of the coating solution is preferably 10xc2x0 C.-20xc2x0 C., more preferably 12xc2x0 C.-18xc2x0 C. The coating solution has a pH of preferably 5.5-7.5, more preferably 6.0-7.0. When the liquid temperature or pH of the coating solution is outside the above-mentioned range, inert particles in the coating solution easily agglomerate, which gives rise to lower productivity due to the clogging of filter in a coating solution circulation system, decreased resistance to falling off of particles from a coating layer and lower time-course stability of the coating solution. It is desirable to filter the coating solution before coating the above-mentioned coating solution, using a filter such as wire-mesh screen, bag type filter, bobbin winder type filter, cartridge type filter and the like, thereby to remove large inert particles that exceed the above-mentioned range of preferable average particle size. The liquid temperature and pH of a coating solution are set within the above-mentioned ranges, and/or the coating solution is filtered using the above-mentioned filter, whereby a coated polyester film having a coating layer surface that shows a 60-degree specular glossiness G1 and a 75-degree specular glossiness G2, both satisfying the above-mentioned formulas (1) and (2), can be easily obtained.
The above-mentioned coating method may be a typical method such as roll coating (e.g., gravure coating, reverse coating, kiss coating, reverse kiss coating and the like), bar coating, air knife method, blade coating, comma coating (roll knife coating), curtain coating, spraying, dipping and the like.
The coating may be applied to the surface of an unoriented polyester film in advance, may be applied to the surface of a monoaxially oriented polyester film, and the film may be further oriented in the direction forming a right angle with the direction of the first orientation, may be applied to the surface of a biaxially oriented polyester film or may be applied in a different manner. Particularly, a method comprising applying the coating to the surface of a monoaxially oriented polyester film and orienting the film in the direction forming a right angle with the direction of the first orientation, thereby to complete crystallization, is most preferable from the aspects of adhesive property, economical benefit, cleanness and the like.